A solid polymer type fuel cell has as a basic structure (unit cell) a proton conductive electrolyte film, catalyst layers forming an anode and cathode arranged sandwiching it between them, gas diffusion layers arranged at the further outsides of the same, and separators arranged at the further outsides of the same. Normally, it is comprised of a plurality of such unit cells connected (stacked) in accordance with the required output.
The principle of power generation of such a solid polymer type fuel cell is as follows: At the gas flow paths of the separators arranged at the two sides of the anode and cathode, the anode side catalyst layer is supplied with hydrogen or another reducible gas while the cathode side catalyst layer is supplied with oxygen or air or another oxidizable gas through the respective gas diffusion layers. When using for example hydrogen gas and oxygen gas as these starting material gases, the following reaction 1 occurs on the catalyst metal of the anode side catalyst layer (oxidation reaction) and the following reaction 2 occurs on the catalyst metal of the cathode side catalyst layer (reduction reaction). The energy difference between these reaction 1 and reaction 2 (potential difference) is utilized to generate power while forming water molecules.H2→2H++2e−(E0=0V)  (reaction 1)O2+4H++4e−→2H2O (E0=1.23V)  (reaction 2)Further, how much voltage is maintained when taking out current to the outside is one indicator of the characteristics of a solid polymer type fuel cell. Normally, if a higher current is taken out, the voltage tends to drop more.
Further, in the catalyst forming the anode and cathode catalyst layers of a solid polymer type fuel cell, usually platinum (Pt) or a platinum alloy having platinum as its main component (Pt alloy) is used as the catalyst metal. Further, to support particulates of such a catalyst metal and, further, enable the generated power to be taken out to an outside circuit, a conductive carbon material is used as the catalyst support.
In this regard, in recent years, soaring prices of precious metals have led to various studies in the field of solid polymer type fuel cells as well on how to make catalyst metals last longer and be more efficient in reaction. For that, however, it is necessary to raise the surface area per unit weight of the catalyst metal contributing to the reaction. Further, for that reason, it is necessary to make the catalyst metal into particulates and have them supported at the support carbon material in a highly dispersed state. However, if this catalyst metal is made too small as particulates, the contact area with the support carbon material will become smaller and, during fuel cell operating conditions, the particulates will fall off from the support carbon surface or will dissolve, precipitate, aggregate, etc. causing problems in durability. As a result, the surface area will become smaller and will no longer be able to contribute to efficient reaction. That is, there is a suitable size for obtaining both durability of the catalyst metal and high efficiency reaction. For example, with platinum metal, a radius 1.5 nm to 5 nm is considered preferable. In actuality, a radius of 3 nm or so is considered ideal. Further, in order for the catalyst metal to remain present in the state of particulates with such optimum size, the particles of the catalyst metal have to be supported at the support carbon material in a highly dispersed state with a certain distance maintained between them. Further, in a catalyst comprised of a support carbon material at which a catalyst metal is supported, to create such an ideal state, it is necessary that the support carbon material have a sufficient specific surface area.
Further, the catalyst layers forming the anode and cathode contain not only the catalyst metal particulates and support carbon material but also, usually, a proton conductive resin for conduction of hydrogen ions (ionomer, below, referred to as “ionomer”). To impart a solid polymer type fuel cell with high cell characteristics, the above reactions 1 and 2 have to be made to proceed as efficiently as possible. For that reason, it is important to raise the proton conductivity in both the anode and cathode catalyst layers and in the proton conductive electrolyte film. That is, the hydrogen ions generated at the anode side catalyst layer move through the water or ionomer of this catalyst layer from the top of the catalyst metal to the inside of the anode side catalyst layer, through the proton conductive electrolyte film, and, further to the inside of the cathode side catalyst layer of the opposing electrode to the top of the catalyst metal of the cathode side catalyst layer. Raising this proton conductivity is important.
In this regard, in general, if the proton conductive electrolyte film and ionomer become dry in state, the proton conductivity will remarkably fall at that dry part. In the operating conditions of a solid polymer type fuel cell, if the inside of the cell is a low humidity state, the proton conductive electrolyte film and ionomer become poor in wet state and a high proton conductivity can no longer be secure. As a result, the conductivity of the hydrogen ions required for above-mentioned electrochemical reaction will become poorer and the power generation efficiency will fall. For example, at the time of small current discharge, the amount of water generated in a cell is small, so the inside of the cell easily becomes a low humidity state and the output voltage sometimes becomes low. For that reason, a solid polymer type fuel cell includes a humidifier set in the system and is operated while maintained in a suitable wet state by operation of this humidifier.
Furthermore, in a solid polymer type fuel cell, to realize high cell characteristics, at the same time as proton conductivity, it is necessary that the starting material gases (reducible gas and oxidizable gas) diffuse in the catalyst layers and continue to be transported to the catalyst metal in fixed amounts. The gas diffusibility of the starting material gases in the catalyst layers is one of the important issues in improving performance (raising output voltage) in high output (large current) operation for practical application of solid polymer type fuel cells. That is, at the time of large current discharge, the above reaction 2 vigorously occurs inside the cathode side catalyst layer, water vapor is generated, and a high humidity state is created, but at this time, the generated water vapor condenses and the condensed water closes the pores in the catalyst layer providing the route for transporting the starting material gases to the catalyst metal. The catalyst metal supported in the closed pores can no longer be supplied with oxygen gas and therefore can no longer contribute to the electrochemical reaction. As a result, the power generation efficiency falls, that is, the so-called flooding phenomenon occurs. To improve the performance under such large current operating conditions, in practical application, suppression of the flooding phenomenon under a high humidity environment is becoming an important issue.
Note that, in the following explanation, “large current” indicates the case where the current value per apparent area of an electrode is 1.5 A/cm2 or so or more. While depending on the flow rate and concentration of the oxygen gas flowing through the cathode, 1.5 A/cm2 is also one metric of the limit current density under common sense operating conditions. Further, regarding the sizes of the pores in the support carbon material of the catalyst forming the catalyst layers, the terms “micropores”, “mesopores”, and “macropores” are used. In accordance with the IUPAC, pore radius 1 nm or less pores are referred to as “micropores”, pore radius 1 to 25 nm pores are referred to as “mesopores”, and, further, pore radius 25 nm or more pores are referred to as “macropores”.
Therefore, in the past, several attempts have been made to improve the gas diffusibility of the starting material gases in the above-mentioned catalyst layers. Several proposals have also been made for tackling the support carbon materials. For example, PLT 1 proposes, as a support carbon material better in gas diffusibility compared with conventional carbon black, carbon black having a primary particle size of a radius of 10 to 17 nm, having secondary particles of primary particles connected together having spaces between them, and having a total volume of radius 10 to 30 nm pores of 0.40 cm3/g to 2.0 cm3/g. Furthermore, large surface area carbon black is increased in pores inside the support disadvantageous to gas diffusibility, so is not suitable for a support carbon material. The BET specific surface area is considered to be preferably 250 to 400 m2/g. However, such a support carbon material has a specific surface area of 400 m2/g or so. This is too small for achieving the practical support rate of catalyst metal of 40 to 70 mass %. For this reason, the particulates of the catalyst metal easily aggregate and as a result the particle size of the supported catalyst metal becomes coarser and it is difficult to prevent a drop in the power generation performance.
Further, PLT 2 proposes a porous carbon material with a total pore volume of 1 ml/g or more and a pore volume of mesopores (=total pore volume-micropore volume (calculated by HK method)) of 50% or more with respect to the total pore volume as an electrode material for electric double-layer capacitor use. Here, if envisioning application of the above porous carbon material excellent in diffusibility of electrolyte ions in an electric double-layer capacitor to a fuel cell, since mesopores are excellent in gas diffusibility, particulates of catalyst metal supported in the mesopores exhibit sufficient catalytic action and therefore improvement of the large current characteristics can be expected. However, on the other hand, micropores also account for tens of percent of the volume. The micropores are easily closed due to the flooding phenomenon, so the particulates of the catalyst metal in these micropores, accounting for a considerable amount of catalyst metal, can no longer contribute to the catalyst reaction at the time of large current and, as a result, a fall in output occurs.
Furthermore, the carbon material for catalyst support use proposed in PLT 3 is a material comprised of so-called dendritic shaped particles comprised of rod-shaped or ring-shaped unit structures connected three-dimensionally. The dendritic parts have lengths of 50 to 300 nm, while the diameters of the dendritic parts are 30 to 150 nm. The spaces formed by these dendritic particles in the catalyst layer contribute to the diffusibility of the reaction gases and reaction product (water). Further, by making the BET specific surface area 200 to 1300 m2/g, it is possible to make the catalyst metal disperse in a highly dispersed state and it is considered a high power generation performance is obtained. However, the carbon material for catalyst support use in PLT 3 has a 0.2 to 1.5 cc/g pore volume in the radius 0.1 to 10 nm region. Micropores, which are easily closed by the water of the reaction product, are present in a certain ratio, so it is difficult to completely prevent the occurrence of the flooding phenomenon.
Furthermore, PLT 4 proposes using, for at least the carbon material in the cathode side catalyst layer, a catalyst support carbon material A giving a relatively stable power generation performance even in the dry state and supporting a catalyst component, a catalyst support carbon material B relatively excellent in gas diffusibility and supporting a catalyst component, a conduction aid carbon material not supporting a catalyst component, and a gas diffusion carbon material low in water vapor adsorption characteristics, hydrophobic, and not supporting a catalyst component, forming a catalyst layer structure making the inside layer at the side contacting the proton conductive electrolyte film a two-phase mixed structure of a catalyst aggregate phase where the catalyst support carbon material A, conduction aid carbon material, and electrolyte material (ionomer) aggregate and a gas diffusion carbon material aggregate phase where the gas diffusion carbon material aggregates and making the outside layer at the side not contacting the proton conductive electrolyte film a two-phase mixed structure of a catalyst aggregate phase where the catalyst support carbon material B, conduction aid carbon material, and electrolyte material (ionomer) aggregate and a gas diffusion carbon material aggregate phase where the gas diffusion carbon material aggregates, and thereby obtaining a fuel cell which, due to the presence of the gas diffusion carbon material condensed phase, is not only excellent in gas diffusibility, but is also resistant to the occurrence of the flooding phenomenon without regard as to the humidity conditions or load conditions and can exhibit high cell performance. In the fuel cell of this PLT 1, excellent gas diffusibility is achieved. In addition, the occurrence of the flooding phenomenon is considerably suppressed. However, the above catalyst support carbon material A has micropores. It is difficult to quickly remove water generated in micropores supporting catalyst metal. The micropores are closed by the water. Further, the catalyst metal in the micropores does not contribute to the reaction. As a result, the reaction efficiency falls by an amount corresponding to the catalyst metal in the micropores, that is, it is difficult to prevent a drop in voltage.